1. The Field of the Invention
The invention relates generally to belt grinders and more particularly to an apparatus and method for grinding a workpiece using an endless abrasive belt and a novel platen design.
2. Description of the Prior Art
Conventional belt grinders typically include an endless abrasive belt mounted between a pair of substantially horizontal, spaced rollers, at least one of which is rotatably driven. The upwardly facing portion of the belt defines a substantially planar grinding surface. To support the workpiece to be ground, a platen is disposed immediately below the grinding surface.
Belt grinders of this design perform relatively well in a wide variety of grinding usages. However, one problem in connection with conventional belt grinders is that there is a great deal of heat which is generated during the grinding process which is not easily dissipated. The heat generated on the ground surface of the workpiece is typically greater at the center than at the periphery so that during the grinding process, uneven thermal expansion of the workpiece often results in more material being ground off near the center of the workpiece than at the periphery. Thus, when the workpiece has cooled, its ground surface will be somewhat concave or dished.
This uneven grinding can be compounded by possible bowing of the platen, also resulting from concentration of heat in the central region thereof.
While this lack of flatness in the ground workpiece often is not great enough to be of concern, it has typically prevented belt grinders from being used for precision grinding operations. One solution is proposed in U.S. Pat. No. 3,801,293 to Kiser, in which a two-stage grinding process is suggested. The first stage consists of achieving a rough grind on the workpiece using a grinder of conventional design. Appreciating that the uneven thermal expansion will cause the ground surface to be dished, Kiser proposes permitting the workpiece to cool after the rough grind and then performing a final, precision grind, utilizing an abrasive belt grinder with a magnetic platen having a magnetic field which increases in strength at its downstream or trailing edge. That is, to avoid the problem common in other types of belt grinders which causes more grinding to take place adjacent the leading edge of the workpiece, Kiser proposes the use of a strong magnetic attraction adjacent the trailing edge of the workpiece. Kiser's second stage grinder has an abrasive belt with a fine grit to minimize the heat in the workpiece and thereby remove the dished out effect caused during the initial rough grinding operation.
While Kiser's design may have a tendency to minimize the uneven grinding caused by unequal thermal expansion of the workpiece, such expansion is still possible due to the absence of any means for dissipating heat from the central portions of the workpiece. Moreover, Kiser's two-stage grinding operation is obviously time consuming and requires two separate belt grinders. Also, because of his magnetic platen, it is only suitable for use with a ferrous metal workpiece. In short, Kiser realized that concentration of heat at the center of the workpiece was the problem, but tried to solve that problem without really addressing the reason for the build up of heat; that is, the lack of sufficient means to dissipate heat at the central regions of the ground surface of the workpiece.
In addition to the uneven grinding resulting from the heating effects of the platen and workpiece, the prior art has not adequately addressed the problem of cavitation which causes undercutting of the workpiece due to air and cooling water collecting between the platen and the abrasive belt. The cavitation results from entrapped air and water creating small bubbles between the belt and platen which concentrates upstream of leading edges or immediately below cavities in the workpiece, causing uneven grinding in these edges of the workpiece. This problem is often aggravated by the accumulation of grinding debris between the belt and platen at these points.
Other prior art belt grinders which are even less pertinent than Kiser's are disclosed in the following U.S. Pat. Nos.: 2,679,128 and 2,586,848 to Miller; 3,266,197 to Olton; 4,290,240 to Robinson; 1,191,045 to Berwer; and 711,397 to Gronvold. The grinders disclosed in the Miller patents, particularly the embodiments of FIGS. 8 and 9 of the '848 Miller patent, are of interest in that they include a reciprocable platen having a plurality of parallel slots obliquely disposed with respect to the direction of travel of the belt. However, these slots are only disclosed as being one of a wide variety of designs possible to provide an impact edge which will cause edge or line contact between the abrasive belt and the workpiece, thereby prolonging the life of the abrasive belt. Thus, Miller's designs are not designed to facilitate cooling of the platen or the workpiece being ground, or to minimize the accumulation of air, water, or grinding debris between the underside of the belt and the platen.
Hence, it is a primary object of the present invention to provide an improved belt grinder which effectively and reliably overcomes the aforementioned drawbacks and limitations of the prior art proposals. More specifically, the present invention has as its objects one or more of the following taken individually or in combination:
1. To provide a belt grinder which, in a single grinding operation, can precision grind a workpiece constructed of either ferrous or nonferrous material;
2. The development of a platen for use with an abrasive belt grinder which incorporates cooling means for cooling and therefore preventing warpage of the platen and any workpiece ground thereby;
3. The provision of an apparatus and method for belt grinding a workpiece which reduces formation of bubbles and accumulation of grinding debris under the belt which can cause uneven grinding in the vicinity of the leading edges of the workpiece; and
4. To develop a platen for an abrasive belt grinder which permits water or any other cooling medium to drain therefrom, thus reducing the possibility of the cooling medium collecting between the platen and the underside of the belt.